Cationic polymerization is an ionic polymerization which proceeds via a stepwise reaction of monomer molecules at positively charged active sites. A number of monomers can be polymerized by this method. Examples of such monomers are olefins, vinyl ethers, vinylarenes such as styrene, but in particular compounds comprising heteroatoms such as ethers, thioethers, oxiranes, oxazolines, esters and acetals. Known initiators for cationic polymerization are protic acids such as perchloric acid or trifluoro-methanesulfonic acid or Lewis acids such as boron trifluoride or aluminum trichloride.
Cationic polymerization has achieved particular importance in relation to the production of polyacetals, which are prepared by polymerization of aldehydes via the opening of the carbonyl double bond or by ring opening of cyclic acetals such as trioxane, on a large industrial scale mostly by bulk polymerization.
However, the known initiators for cationic polymerization generally have certain disadvantages. Boron trifluoride is typically stored as a gas under high pressure and is very difficult to handle. Similarly, the quality of polyacetals produced therewith, in particular their long term stability, still leaves room for improvement.
DE 2141600 describes a process for homo- and copolymerization of trioxane in the presence of trifluoromethanesulfonic acid and its homologs with 1-18 carbon atoms as initiator.
EP 0 678 535 describes the production of polyoxymethylene copolymers by means of cationic polymerization using trifluoromethanesulfonic acid and its homologous acids and anhydrides as initiators. The initiator is used in a concentration range of 5*10−6 to 2*10−5 mol %, based on the main monomer. The initiators of the cited prior art are extremely active, but have for that very reason the disadvantage that even small dosage fluctuations can lead to pressure fluctuations in the polymerization reactor. In addition, these active initiators react very sensitively to even small amounts of impurities in the monomer, which has a direct effect on process stability. Consequently, these initiators are currently not being used industrially despite their high activity: they do not meet the requirements of commercial manufacture in terms of reliability and production consistency.